Coextrusion process for producing multiple layered thermoplastic pipe

ABSTRACT

A process is disclosed for the coextrusion of a multi-layered tube whereby at least one stream of molten polymer from a single-screw extruder is passed through a die; wherein the stream is divided into two concentric streams which are balanced by varying the area of a restrictor orifice placed in the central stream. Spider lines in the outer molten stream are eliminated by the use of a second restrictor means used downstream of an open chamber for reuniting the melt, which chamber is downstream of the spider legs.

BACKGROUND OF THE INVENTION

The present invention relates to a process for producing multiplelayered pipe, preferably of two or more thermoplastic resins, andpreferably having three layers comprising at least one outer layer, anintermediate film layer, and at least one inner layer.

Many attempts have been made to produce a multiple layered pipe in orderto combine the desirable characteristics of different thermoplasticresins. These attempts have usually involved extruding several streamsof thermoplastic resins through a series of concentric tubes which arefixed relative to one another in a radial manner, such as by spiders, todefine annular passages therebetween, and then subsequently joining theresultant annular layers to produce a multiple layered pipe. Pipesprepared by such processes and apparatus, however, have had spider marksthereon due to the flow interruptions caused by the many spidersnecessitated in such apparatus. Moreover, such apparatus containadjusting screws which protrude into the individual die passages foradjusting the thickness of the annular layers. These screws alsointerfere with the flow of the molten resin as it is being extrudedcausing additional marks on the pipe.

Furthermore, it is undesirable to provide an extrusion die for eachlayer of the desired pipe. Since high internal pressure is required forthe extrusion of the highly viscous heat plastified material, suchapparatus are subject to distortions in the die, and multiple-dieprocesses may result in undesirable nonuniformity in the thickness ofeach layer, unless the extrusion pressures of each layer are balanced.However, in practice, it is frequently difficult to maintain extrusionpressures constant in their proper relationship. Those processesemploying a separate die for each layer thus inherently involvedifficulty in controlling the thickness of each layer in the desiredpipe.

Another disadvantage associated with conventional processes for theproduction of co-extruded plastic pipe is their inability to equalizethe flow of thermoplastic within the die so that the heat plastifiedmaterial is applied as a continuous layer of constant thickness. Thisproblem is particularly prevalent when layers of thermoplastic areextruded through radial orifices. In order to form a layer of constantthickness, it is absolutely essential that the flow of thermoplasticthrough the orifice is constant along its entire circumference.

In the extrusion of multiple layered pipe having an intermediate thinfilm layer of thermoplastic material, it is necessary at the point oflaydown that the layers of thermoplastic material have viscosity indexessimilar to that of the film layer. Any significant difference inviscosity will prevent bonding of the outer and inner layers to thefilm. In addition to controlling the relative viscosities, it is alsonecessary that each layer have a constant thickness. Failure to achievevery precise viscosity control and uniformity of thickness of each layerin pipe having one or more thin film layers, results in a product whichhas inferior physical properties.

U.S. Pat. No. 4,249,875 by Hart et al discloses a process and apparatusfor producing multiple layered pipe utilizing a single annular diepassage to laydown multiple inner and/or outer layers of thermoplasticonto a main stream of thermoplastic. An extrusion passage of suitablelength was utilized to allow the bonded thermoplastic to reunite fromthe interruptions caused by the radial support legs of the mandrelpositioned in the extrusion passage. This reuniting of the thermoplasticoccurred at the same time as the laydown of the inner and outer layers,resulted in some lack of uniformity in layer thickness. In addition, theHart process utilizes separate extruders to supply thermoplasticmaterial to the inner and outer layers respectively. The use of separateextruders makes it difficult to control the relative viscosities of theinner and outer layers because of the difficulty of operating bothextruders at the same temperature.

It would be desirable, therefore, if a process for the production ofmultiple layered pipe having an intermediate film layer was availablewhich could attain very precise control of both the layer thickness andviscosity of the thermoplastic throughout the narrow extrusion passagesof the die.

SUMMRAY OF THE INVENTION

In accordance with the present invention there is provided a process formaking multiple layered thermoplastic pipe. The process comprisestransporting a first portion of an annular stream of a firstthermoplastic material through a main extrusion passageway having anannular restriction within. The annular restriction reunites thethermoplastic material after flow disruptions of the thermoplasticmaterial caused by support elements projecting into the main extrusionpassageway. The process also comprises transporting a second portion ofthe first thermoplastic material through a coextrusion passagewayaxially disposed within the annulus formed by the main extrusionpassageway, and balancing any difference in flow rates between thethermoplastic material in the main extrusion passageway and thethermoplastic material in the coextrusion passageway by restricting theflow of the thermoplastic material through the coextrusion passageway.Flow restriction in the coextrusion passageway may be accomplished byinterchangeable restrictor elements having different sized orifices. Inaddition, the process comprises applying an annular stream of the secondportion of the first thermoplastic material to the inside surface of theannular stream of the first portion flowing over the second portion. Theabove process will produce a multi-layered thermoplastic pipe having twolayers of a similar thermoplastic material.

Another embodiment of the present invention comprises the process formaking multiple layered thermoplastic pipe comprising transporting afirst portion of an annular stream of a first thermoplastic materialthrough a main extrusion passageway having an annular restrictionwithin, and then reuniting the thermoplastic material after flowdisruptions of the thermoplastic caused by elements projecting into themain extrusion passageway. The process also comprises transporting asecond portion of the first thermoplastic material through a coextrusionpassageway axially disposed within the annulus formed by the mainextrusion passageway, and balancing any difference in flow rates betweenthe thermoplastic material in the main extrusion passageway and thethermoplastic material in the coextrusion passageway by restricting theflow of thermoplastic through the coextrusion passageway.

The process also comprises applying an annular stream of a secondthermoplastic material to the inner surface of the annular stream of thefirst portion of the first thermoplastic material flowing over thesecond thermoplastic material. The stream of the second thermoplasticmaterial is applied downstream of the main extrusion passageway annularrestriction. In addition, the process comprises applying the annularstream of the second portion of the first thermoplastic material to theinside surface of the annular stream of the second thermoplasticmaterial flowing over the second portion. The annular stream of thesecond portion of the first thermoplastic material is applied downstreamof the application of the second thermoplastic material. An additionalembodiment comprises applying at least one additional annular stream ofthermoplastic material to the inside surface of the annularthermoplastic stream flowing thereover. The additional annular stream isapplied downstream of the application of the second thermoplasticmaterial but before application of the second portion of the firstthermoplastic material.

Still another embodiment of the present invention comprises a processfor making multiple layered thermoplastic pipe comprising transporting afirst portion of an annular stream of a first thermoplastic materialthrough an annular main extrusion passageway and directing it into anannular restriction therein to reunite the thermoplastic material afterflow disruption of the thermoplastic material caused by support legs or"spiders" projecting within the main extrusion passageway. The processalso comprises transporting the second portion of the firstthermoplastic material through a coextrusion passageway axially disposedwithin the annulus formed by the main extrusion passageway, andbalancing any difference in flow rates between the thermoplasticmaterial in the main extrusion passageway and the thermoplastic materialin the coextrusion passageway by varying the flow of thermoplasticthrough the coextrusion passageway through the use of interchangeablerestriction orifices. In addition, the process comprises applying anannular stream of a second defined material to the inner surface of theannular stream of the first portion of the thermoplastic materialflowing over the second defined material. The second defined material isapplied downstream of the main extrusion passageway annular restriction.Furthermore, the process comprises applying an annular stream of a thirdthermoplastic material to the inner surface of the annular stream of thesecond defined material flowing over the third thermoplastic material.The third thermoplastic material is applied downstream from theapplication of the second thermoplastic material. The process alsocomprises applying an annular stream of a fourth defined material to theinner surface of the annular stream of the third thermoplastic materialflowing over the fourth defined material. The fourth defined material isapplied downstream from the application of the third thermoplasticmaterial. Additionally, the process comprises applying comprisesapplying an annular stream of the second portion of the firstthermoplastic material to the inner surface of the annular stream of thefourth defined material flowing over the second portion. The secondportion of the first thermoplastic material is applied downstream of theapplication of the third thermoplastic material. The second and fourthdefined materials may be an adhesive or a thermoplastic resin.

Various other features and attendant advantages of this invention willbe more fully appreciated from the following detailed description of thepreferred embodiments when considered in connection with theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an axial sectional view of a three layer pipe;

FIG. 2 is a perspective view of one embodiment of an apparatus forpracticing the present invention showing the attachment of the mainextruder, coextruder, vacuum sizer, pulling means and rotary scanner inrelation to a coextrusion die adapted for the formation of a three layerpipe;

FIG. 3 is a enlarged longitudinal section through the die apparatus ofFIG. 2;

FIG. 4 is an axial sectional view along the left end face of the dieinlet body and mandrel cone section taken on a line 4--4 of FIG. 3;

FIG. 5 is an axial sectional view taken through the spider body andmandrel spider section taken on a line 5--5 of FIG. 3;

FIG. 6 is a partial axial sectional view taken along the right end faceof the mandrel first laydown section taken on a line 6--6 of FIG. 3;

FIG. 7 is an axial sectional view of a five layer pipe;

FIG. 8 is a perspective view of other apparatus for performing thepresent invention showing the attachment of the main extruder,coextruder, vacuum sizer, pipe pulling means and rotary scanner inrelation to a die apparatus adapted for the formation of the five layerpipe;

FIG. 9 is a longitudinal section through the die apparatus taken alongline 9--9 of FIG. 8; and

FIG. 10 is an axial sectional view through the spider body and spidersection taken along line 10--10 of FIG. 9.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring now to the drawings; FIG. 1 illustrates one embodiment of theprocess of the present invention comprising a three layeredthermoplastic pipe. The pipe indicated at 14 has three layers, a barrierfilm middle layer 18 having a thickness from about 5 to about 10 mils,an inner layer 16 having a thickness greater than about 20 mils, and anouter layer 19 having a thickness greater than about 20 mils. It shouldbe appreciated that the thicknesses of the outer layer 19 and innerlayer 16 will depend on the desired total wall thickness of the pipe.

The pipe 14 is produced by the process of the present invention on theapparatus illustrated in FIG. 2. The die apparatus indicated at 20includes an annular transition body 22 and an annular discharge sleeve24. Interposed between sections 22 and 24 is an annular die inlet body23 positioned adjacent to and downstream of transition body 22; anannular spider body 26 positioned adjacent to and downstream of dieinlet body 23, and an annular sleeve adjustment body 28 positionedadjacent to and downstream of spider body 26. Transition body 22 isconnected to inlet body 23 by connector ring 25. A main extruder 32,including feed hopper 33 is connected to die apparatus 20 at transitionbody 22 by feed conduit 34. Control valve 36 is used to adjust the rateof feed of material through conduit 34. A coextruder 40 including feedhopper 41 is connected to die apparatus 20 at spider body 26 by feedconduit 42. Control valve 44 regulates the rate of flow through conduit42. A heat exchanger (not shown) may be positioned between main extruder32 and die apparatus 20 in communication with feed conduit 34 to obtainmore precise control over the temperature of the material exiting frommain extruder 32.

Also shown in FIG. 2 is a vacuum sizer 48 which preferably is utilizedin conjunction with the die apparatus to perform the present inventionfor controlling the diameter of the pipe 14. The vacuum sizer 48 isconstructed to contain in approximately the first half of its length, asizing die 52 in which the extruded pipe 14 passes immediately after itsexit from discharge sleeve 24 through discharge passage 62 (FIG. 3). Thesizing die 52 functions to adjust the extruded pipe to its finishedsize. The latter half of the vacuum sizer 48 is constituted by a coolingchamber (not shown) also maintained under vacuum. In this chamber, thefinal extruded and sized pipe is cooled as nearly as possible to ambienttemperature in order to further solidify the thermoplastic materials ofeach of the layers of the pipe 14. Cooling is suitably achieved bycontacting the extruded article with a fine spray of cooling water.Alternatively, cooling may be achieved by immersing the pipe 14 in awater bath.

Downstream of the vacuum sizer 48 is a pipe pulling means 53 for pullingthe pipe 14 from the sleeve 24 through the vacuum sizer 48 and along theapparatus line. Pipe pulling means 53 may comprise a pair of opposingrotary treads 54 which engage the pipe 14 therebetween to imparthorizontal movement to it. The rotational velocity of treads 54 may beadjusted to increase the horizontal travel of pipe 14 forming indischarge sleeve 24, thereby causing the pipe 14 to stretch axiallybetween die apparatus 20 and vacuum sizer 48, and also reducing the wallthickness of pipe 14 about 12 to 15%. This linear stretching, whichdecreases the wall thickness of pipe 14, prepares the pipe for entryinto vacuum sizer 48.

Downstream of pipe pulling means 53 is a rotary scanner 55 such as theone manufactured by NDC Systems utilizing probe 51 which rotates 360°about the surface of pipe 14 to continuously display and record the wallthickness of pipe 14.

Referring now to FIG. 3 which illustrates a longitudinal cross-sectionof die body 20, there is a transition die body 22 at the left endthereof and a discharge sleeve 24 at the right end. Interposed betweendie sections 22 and 24 are inlet body 23, spider body 26, and sleeveadjustment body 28 held together in end-to-end relationship by bolts 54engaged in threaded holes 56. Discharge sleeve 24, concentricallypositioned within sleeve adjustment body 28, includes an annular ridge57 about the circumference thereof which is engaged by annular lip 59 ofsleeve adjustment body 28 to hold the discharge sleeve 24 against spiderbody 26. The outer diameter of discharge sleeve 24 is smaller than theinside diameter of sleeve adjustment body 28 to allow sleeve 24 fromabout 5 mils to about 7 mils radial movement therein. Circumferentiallypositioned about sleeve adjustment body 28 are a plurality of sleeveadjusting bolts 61 located to radially align sleeve 24 within adjustmentbody 28.

Longitudinally traversing through die body 20 is a die bore indicated at58 including an inlet passageway 60 at the left end of die apparatus 20and outlet passageway 62 at the right end thereof. A mandrel indicatedat 66 is longitudinally disposed inside bore 58 to define an annularextrusion passageway 68 formed between the outer surface of mandrel 66and the inner surface of die apparatus 20. The mandrel 66 isequidistantly supported within bore 58 by spider legs 70 radiallyextending from the inner surface of spider body 66 into bore 58. Mandrel66 includes a conical nose 72 positioned in close proximity to inlet 60of bore 58. Inlet 60 is in direct communication with main extruder feedline 34. Inlet 60 narrows prior to conical nose 72 to provide atransition zone from the larger main extruder discharge line 34. Aroundconical nose 72, inlet 60 diverges in spaced relationship to conicalnose 72, thereby defining an extrusion inlet passageway 74.

Also included in mandrel 66 is a central feed passage 76 axiallydisposed therein and in direct communication with inlet 60 through thetip of conical nose 72. Conical nose 72 (further illustrated in FIG. 4)includes a central feed passage restrictor 82 comprising a threaded plug84 and a constrictor orifice 86 extending therethrough, communicatingfeed passage 76 with inlet 60. Threaded plug 84 includes squarereceptacle 85 for the receipt of an Allen wrench for removal of plug 84and replacement by other plugs having orifices of different diameters.

Referring again to FIG. 3, mandrel 66 includes at the right end thereof,positioned within die discharge sleeve 24, a cylindrically-shapedmandrel discharge section 90. Positioned between conical nose 72 anddischarge section 90 are spider section 94, first laydown section 96 andsecond laydown section 98 which are joined in end-to-end relationship bya bolt 79 axially centrally disposed therethrough. Washer 81 and nut 82at the discharge end of discharge section 90 secure the mandrel sectionson bolt 79. The discharge portion 62 of extrusion passageway 68 asdefined by discharge sleeve 24 and discharge section 90 determines boththe total wall thickness of the thermoplastic pipe and its diameter.Therefore both sleeve 24 and discharge section 90 are removable whenrequirements dictate a pipe having a different wall thickness and/ordiameter. Central feed passageway 76 is axially disposed within bolt 79through laydown sections 96, 98, and discharge section 90.

Spider section 94, in addition to feed passage 76 extendingtherethrough, includes a longitudinal feed passage 106 comprising aslotted passage (see FIG. 5) longitudinally extending through spidersection 94 and terminating in a reservoir chamber 104 milled into theright end face of spider section 94 and annularly disposed about bolt79. In direct communication with longitudinal feed passage 106 is inletpassage 108 passing vertically through spider body 26 and spider leg 70and in direct communication with feed conduit 42 through feed connector109 for the transport of feed from coextruder 40 to die 20.

Abutting spider section 94 in a direct end-to-end relationshipdownstream thereof is first laydown section 96. First laydown section 96includes annular chamber 112 milled into the right end face thereof andannularly disposed about bolt 79. Also included therein are individualfeed equalizer ports 116 (see FIG. 6) located parallel to bolt 79 andcircumferentially disposed about bolt 79 in direct communication withannular reservoir chamber 104. Also milled into the right end face offirst laydown section 96 and further defined by the left end face ofsecond laydown section 98 is annular distribution orifice 118 annularlydisposed within first laydown section 96 and providing a restrictedpassageway between annular feed chamber 112 and extrusion passage 68 toinsure a uniform circumferential laydown of thermoplastic material.

Located downstream of the first laydown section 96 directly adjacentthereto in end-to-end relationship is second laydown section 98.Included in second laydown section 98 is annular reservoir chamber 124comprising an annular space milled therein in direct communication withcentral passageway 76 via a plurality of radial distribution holes 125formed through bolt 79. At the right end face of second laydown section98 is annular feed chamber 126 milled therein and in communication withannular reservoir chamber 124 through equalizer ports 128. Also milledinto the right end face of second laydown section 98 and further definedby the left end face of discharge section 24 is annular distribution or"laydown" orifice 130 connecting annular feed chamber 126 with extrusionpassageway 68.

An important part of the inventive process is the use of a constriction138 in extrusion passageway 68 located between spider legs 70 and thefirst laydown orifice 118. As shown in FIG. 3, extrusion passagewayconstriction 138 is formed by the convergence of the side walls ofmandrel 66 and spider section 94. The purpose of the extrusionpassageway constriction 138 is two-fold, (1) to reunite thethermoplastic material in extrusion passage 68 after it has beenseparated by the radial spider legs 70, and (2) to relieve the backpressure in main extrusion passageway 68 downstream of laydown orifice130 to allow easier and more precise radial adjustment of dischargesleeve 24. The diverging portion 139 of constriction 138 formed bylaydown section 96 and discharge sleeve 24 is sufficiently wide tomaintain communication of thermoplastic melt through passageway 68during radial re-positioning of sleeve 24. Other embodiments attemptedto utilize constrictions downstream of the last laydown orifice to helpreknit the layer of thermoplastic separated by spider legs 70, but theback pressure created in passageway 68 by the constriction wassufficient to interfere with radial adjustment of discharge sleeve 24.It should be appreciated, however, that the pressure drop in extrusionpassage 68 across discharge sleeve 24 should be between about 400 and600 psi to assist in the bonding together of the individual layers ofthe pipe. In addition, the diameter of constriction 138 should be suchthat a pressure drop of from about 400 to about 600 psi acrossconstriction 138 is achieved. Generally, such a pressure drop acrossconstriction 138 is achieved when forming pipe having diameters of from2 to 6 inches, by limiting the total cross sectional area of extrusionpassage 68 at constriction 138 to about one square inch. Generally,constriction 138 should be at least one inch in length at its narrowestpoint to ensure adequate rejoining of the thermoplastic melt.

Referring first to FIG. 2, the process proceeds as follows:thermoplastic material is loaded into feed hopper 33 of main extruder 32and formed into a melt state therein. The main extruder 32 andcoextruder 40 may each be a screw extruder or rotary extruder of adesign which is familiar to those persons skilled in the art. Theprocess may be performed on the apparatus described herein to produce amultiple layered pipe with a wide range of thermoplastic materials,including all extrudable plastic materials. Examples of such materialsinclude cellulose esters and ethers such as ethyl cellulose acetate,vinyl and vinylidene polymers and copolymers such as polymers andcopolymers of vinyl chloride, vinyl acetate, vinylidene chloride, ethylvinyl alcohol, polyvinyl alcohol; and polymers and copolymers of acrylicand methacrylic esters; polymers and copolymers of olefins, such asethylene, propylene, and butylene; polymers and copolymers of styrene,2-methyl styrene and their mixtures of elastomeric copolymers;polyamides; polycarbonates; polyaldehydes, polyethers; polyurethanes;polyesters; natural and synthetic elastomers; and silicon resins andelastomers and polyethylene terephthalate. Preferably, however,polyethylene terephthalate, ethylene vinyl alcohol, or the polystyrenesand their copolymers and elastomers are employed, such as polystyrene,styrene-acrylonitrile-copolymers (SAN),styrene-butadiene-acrylonitrile-copolymers (ABS), andmethacrylate-styrene-rubber copolymers.

These plastic materials can, of course, be used in admixture withfillers, plasticizers, colorants, or other ordinary additives, providedthey are in a state permitting melt extrusion.

In one preferred embodiment, moreover, these plastics are combined totake advantage of the desirable properties of each thermoplastic. By wayof example of desirable properties, there may be mentioned mechanicalstrength, resistance to shock, thermal properties, transparency,opacity, resistance to chemicals, impermeability to liquids, gases, andodors, ease of working, ability to receive printing or decoration, etc.Particularly preferred according to the present invention is a threelayered pipe having an outer layer of polyethylene terephthalate, anintermediate barrier film of ethyl vinyl alcohol, and an inner layer ofpolyethylene terephthalate.

After discharging from extruder 32 through line 34, the melt enters diebody 20 through inlet 60. Referring now to FIG. 3, the thermoplasticmelt then flows through inlet 60 where it subsequently impacts conicalnose 72 of mandrel 66. A portion of the thermoplastic melt continuesaxially through restrictor orifice 86 and into axial passageway 76through radial orifices 125 to annular reservoir chamber 124 of thesecond laydown body 98. The remaining melt impacts conical nose 72 andis axially pierced, radially separated, and directed through extrusioninlet passageway 74 into extrusion passageway 68. It should beappreciated that upon entering extrusion passageway 68, the melt willcontact spiders 70 located therein causing the thermoplastic melt toseparate as it flows around the spider legs 70. In conventionalprocesses, the spider legs were positioned at a point early in theextrusion process before a multiple layered annular stream was formed.This provided a length of uninterrupted extrusion passageway afterlaydown of the multiple layers to allow the objectionable marks createdby the spiders to be eliminated by the melt being reunited. Generally,the uninterrupted passageway was provided through use of a mandrel andsurrounding sleeve of suitable length to allow the bonded layers ofthermoplastic to stabilize before exiting the die body. It has beenfound that "reknitting" or rejoining a thermoplastic outer layer whichis part of a multi-layered stream creates a certain lack of uniformityin the layer thickness. In the present invention, however, extrusionpassageway 68 has been constricted subsequent to or downstream of spiderlegs 70, but prior to the laydown of the multiple layers. It has beenfound that through the use of constriction 138, that the reknitting ofthe thermoplastic melt and elimination of the objectionable spider marksis better accomplished prior to the formation of the multiple layers.

Working in combination with main extruder 32 is coextruder 40 depictedin FIG. 2, which receives thermoplastic material through feed hopper 41and transforms it into a melt. The thermoplastic melt exits coextruder40 through feed line 42 and control valve 44 into spider body 26.

Referring now to FIG. 3, die spider body 26 receives the thermoplasticmelt through line 42 to a feed connector 109, having a passagewaytherein in direct communication with inlet passageway 108. Thethermoplastic melt continues through passageway 108 into longitudinalfeed passage 106 and into annular reservoir chamber 104. In annularreservoir chamber 104 the flow rate of thermoplastic melt is decreasedand uniformly distributed about reservoir chamber 104 by the backpressure from feed equalizer ports 116 caused by the smaller diameterthereof. It should be remembered that longitudinal feed passage 106 isoversized to allow an increased flow rate of thermoplastic material intoreservoir chamber 104. Feed equalizer ports 116 are sized sufficientlynarrow to create a back pressure or pressure differential thereacross.The decreased flow rate of thermoplastic through feed ports 116 providesa sufficient pressure differential to ensure complete distribution ofthermoplastic material about the annulus of reservoir chamber 104. Themelt is then uniformly distributed through feed ports 116 to annularfeed chamber 112. From annular feed chamber 112 the thermoplastic meltflows outwardly through annular laydown orifice 118 into extrusionpassageway 68. The use of feed equalizer ports 116 to equalize flow tothe annular feed ring is further described in U.S. Pat. No. 4,249,875incorporated herein by reference. As the melt from laydown orifice 118enters extrusion passageway 68 it is laid down on the inner surface ofthe stream of thermoplastic melt entering extrusion passageway 68through feed passage 74 thereby forming a two layer thermoplastic flowcomprising outer layer 19 and middle layer 18 of pipe 14 (FIG. 1).

Thermoplastic melt from main extruder 32 flows through restrictororifice 86 and central feed passage 76 into annular reservoir chamber124 of the second laydown body 98. The annular reservoir chamber 124acts in a manner similar to annular reservoir chamber 104 to equalizeand distribute the thermoplastic melt to feed equalizer ports 128. Fromfeed equalizer ports 128 the thermoplastic melt flows axially to annularfeed chamber 126 where it is radially distributed through annularlaydown orifice 130 into extrusion passageway 68, where it lays down onthe inner surface of the two layer thermoplastic melt flowing downstreamtherein to form inner layer 16 of pipe 14.

It should be appreciated that constriction 138 will create a lowpressure area downstream thereof in extrusion passageway 68 due to thereduced area for flow of thermoplastic melt through the constriction.Due to the pressure imbalance therein the flow of melt through extrusionpassage 68 will decrease, resulting in a corresponding increase in theflow of thermoplastic through central passageway 76. In order to remedythe flow rate imbalance, a central feed passage restrictor 82 is placedin the mouth of bore 76 to equalize the flow rates in bore 76 andextrusion passageway 68. The diameter of restrictor orifice 86 willdepend on several factors, including among others, the width ofextrusion passageway 68, the diameter of bore 76, and the viscosity ofthe thermoplastic melt. Any final adjustments of restrictor orifice 86will be based on trial and error by observing the layer thicknessesdisplayed and recorded by scanner 55. If, after observing the readingsfrom the rotary scanner 55, it can be seen that the thickness of outerlayer 19 is too small in relation to inner layer 16, a plug 84 having asmaller orifice should be substituted to reduce the flow in centerpassage 76. On the other hand, if outer layer 19 is too thick, then aplug 84 having a larger orifice is necessary. Additional adjustments inthe thickness of outer layer 19, inner layer 16 or in the concentricityof pipe 14 may be performed by rotating adjusting bolts 61 to radiallyreposition sleeve 24 in the desired location.

Another significant aspect of the present invention is the laydown ofinner layer 16 and outer layer 19 from a single main extruder byutilization of central passageway 76 through mandrel 66. It should beappreciated that when coextruding material onto a thin intermediatebarrier film it is important that the relative viscosities of the filmlayer and outer/inner layers be compatible. Utilizing a single extruderto form both layers not only decreases the cost; it also allows a moreuniform temperature to be maintained in the thermoplastic of outer layer19 and inner layer 16. Uniform temperatures in layers 19 and 16 allowgreater control over their relative viscosities to obtain better bondingand more uniform thickness of these layers. Such uniformity intemperature is not otherwise obtainable when utilizing separateextruders for each layer.

In another embodiment of the present invention, a five layer pipe isformed as illustrated in FIG. 7. The pipe indicated at 148 includes aninner layer 150 of a thermoplastic material, a second layer 152 of adefined material such as adhesive, a center or barrier film layer 154 ofthermoplastic, a fourth layer 156 of a defined material such asadhesive, and an outer layer 158 of thermoplastic. Layers 150 and 158preferably have thicknesses greater than about 20 mils; barrier layer154 will have a thickness from about 5 to about 10 mils; the thicknessesof each layer in FIG. 7 not being shown in proportion, however. Itshould be appreciated that in the five layer pipe, two layers ofadhesive are necessary when outer layer 158 and film layer 154 are notcompatible for being directly bonded to one another during coextrusion,whereas in the three layer embodiment illustrated in FIG. 1 no adhesivewas necessary since the outer, center and inner layers were made ofthermoplastic capable of being bonded by coextrusion. A method forforming a five layer pipe having five layers of compatible thermoplasticmaterial joined without the use of adhesive is also contemplated by thepresent invention.

Materials useful in the formation of the pipe in this embodiment havebeen described previously. Adhesives useful in the present inventioninclude those commercial products designed for bonding of polymerlayers.

In one preferred embodiment outer layer 158 and inner layer 150 arepolyethylene terephthalate, the barrier film layer 154 is ethylene vinylalcohol, and the adhesive layers 152, 156 are DuPont CXA 136. In anotherpreferred embodiment, outer layer 158 is polyethylene terephthalate, thefilm layer 154 is polystyrene and the adhesive layers are DuPont 1123.

The apparatus used to perform the process of this invention to form thefive layer thermoplastic pipe is illustrated in FIG. 8. The apparatusshown in FIG. 8 is similar to that shown in FIG. 1 except that in FIG. 8die apparatus 20 is replaced by a larger die apparatus indicated at 162and having additional laydown means therein. Die apparatus 162 includesa die transition body 165 at the left end thereof and a die dischargesleeve 170 at the right end thereof. Interposed between die bodies 165and 170 are die inlet body 168, spider body 166, and a discharge sleeveadjustment body 172. In addition, a second coextruder 38 having a feedline 43 and control valve 45 is connected to spider body 166. Coextruder38 contains adhesive material which is fed through hopper 39.

As can be seen from FIGS. 9 and 10, a mandrel 182 is supported in bore58 by spider body 166 at mandrel spider section 186. Spider mandrelsection 186 includes a plurality of outer longitudinal passageways 187and a plurality of inner longitudinal passageways 189 radially offsettherefrom. Passageways 187, 189 are concentrically located about bore 58in communication with passageway 68 and inlet 74 for the passage ofthermoplastic material therethrough. By utilizing spider section 186,mandrel 182 is held in place about its entire circumference to providegreater support within bore 58. Also included in mandrel spider section186 is the continuation of axial bore 76, and a feed passage 190beginning at feed connector 144 at the outer surface of spider die body166, traversing vertically through spider body 166 and continuing firstvertically, then axially through spider section 186. A seconddistribution passageway 192, begins at feed connector 145 located at thesurface of spider body 166 and vertically passes through spider body 166to communicate with an annular reservoir chamber 194 milled into theright end surface of spider section 186. Annular reservoir chamber 194is further defined by the left end face of first laydown section 196.Feed passageways 190, 192 are in direct communication with feed conduits42, 43 respectively through feed connectors 144, 145 respectively.

Adjacent to and downstream of spider section 186 is first laydownsection 196 including therein a continuation of central feed passage 76,a continuation of feed passage 190, a feed passage 200 in communicationwith annular reservoir chamber 194, and feed equalizer ports 202 whichcommunicate with annular feed rings 204 and annular chamber 194. Annulardistribution or "laydown" orifice 206 is in direct communication withextrusion passageway 68 and annular feed ring 204 for laydown of anouter adhesive layer 156 onto the outer thermoplastic layer 158 comingfrom main extruder 32. Adhesive is introduced into passageway 68 throughannular laydown orifice 206, however thermoplastic may be substitutedtherefor when adhesive bonding is not necessary between layers. Itshould be appreciated that passages 190 and 192 must be radially offsetfrom central passage 76. All passages are shown lying in the plane ofFIG. 9 for purposes of convenience however.

Returning now to FIG. 9, second laydown section 211 adjacent to anddownstream of first laydown section 196 includes a continuation ofcentral feed passage 76 and a continuation of passageway 200. Alsoincluded therein is feed passage 190 which communicates with annularreservoir chamber 210 through radial distribution ports 125. Reservoirchamber 210 communicates with annular feed ring 212 through feedequalizer ports 214. Annular feed ring 212 is in direct communicationwith extrusion passageway 68 through annular laydown orifice 216 toprovide a tubular stream of thermoplastic material forming the filmbarrier layer 154.

Downstream and adjacent to second laydown section 211 is third laydownsection 220. Third laydown section 220 includes a continuation of axialfeed passage 76, and an annular reservoir chamber 222 in communicationwith feed passage 200 through radial distribution ports 221. Annularfeed ring 224 communicates with annular reservoir chamber 222 throughfeed equalizer ports 228. Annular laydown orifice 230 in communicationwith annular feed chamber 224 and extrusion passageway 68 provides atubular laydown of the stream comprising adhesive layer 152.

Adjacent and downstream of third laydown section 220 is fourth laydownsection 232 which includes therein annular reservoir chamber 234, radialdistribution ports 235, feed equalizer ports 236, annular feed chamber238 and annular laydown orifice 240 which are in communication withcentral passageway 76 to provide the laydown of the stream comprisinginnermost thermoplastic layer 150.

Although several specific preferred embodiments of the present inventionhave been described in the detailed description above, this descriptionis not intended to limit the invention to the particular form orembodiments disclosed herein since they are to be recognized asillustrative rather than limitative, and it will be obvious to thoseskilled in the art that the invention is not so limited. For example,even though a process has been described showing the laydown of three orfive layer pipe, it should be appreciated that a seven or more layerpipe may be formed by the use of additional laydown means. Thus, theinvention is declared to cover all changes and modifications of thespecific examples of the invention herein disclosed for purposes ofillustration which do not constitute departures from the spirit andscope of the invention.

The embodiments of the invention in which an exclusive property orprivilege is claimed are defined as follows;
 1. A process for makingmultiple layered thermoplastic pipe, comprising:(a) transporting a firstportion of an annular stream of a first thermoplastic material throughan annular main extrusion passageway having an annular restrictiontherein, the annular restriction reuniting the thermoplastic materialafter flow disruption thereof; (b) transporting a second portion of thefirst thermoplastic material through a coextrusion passageway axiallydisposed within the annulus formed by the main extrusion passageway; (c)balancing any difference in flow rates between the thermoplasticmaterial in the main extrusion passageway and the thermoplastic materialin the coextrusion passageway by restricting the flow of thermoplasticthrough the coextrusion passageway; (d) applying an annular stream of asecond thermoplastic material to the inner surface of the annular streamof the first portion of the first thermoplastic material flowingthereover, the stream of the second thermoplastic material applieddownstream of the main extrusion passageway annular restriction; and (e)applying the annular stream of the second portion of the firstthermoplastic material to the inside surface of the annular stream ofthe second thermoplastic material flowing thereover, the annular streamof the second portion of the first thermoplastic material applieddownstream of the application of the second thermoplastic material. 2.The process of claim 1 comprising applying at least one additionalannular stream of thermoplastic material to the inside surface of theannular thermoplastic stream flowing thereover, the additional annularstream applied downstream of the application of the second thermoplasticmaterial but before application of the second portion of the firstthermoplastic material.
 3. A process for making multiple layeredthermoplastic pipe comprising:(a) transporting a first portion of anannular stream of a first thermoplastic material through an annular mainextrusion passageway having an annular restriction therein, the annularrestriction reuniting the thermoplastic material after flow disruptionthereof; (b) transporting a second portion of the first thermoplasticmaterial through a coextrusion passageway axially disposed within theannulus formed by the main extrusion passageway; (c) balancing anydifference in flow rates between the thermoplastic material in the mainextrusion passageway and the thermoplastic material in the coextrusionpassageway by restricting the flow of thermoplastic through thecoextrusion passageway; (d) applying an annular stream of a seconddefined material to the inner surface of the annular stream of the firstportion of the first thermoplastic material flowing thereover, thesecond defined material applied downstream of the main extrusionpassageway annular restriction; (e) applying an annular stream of athird thermoplastic material to the inner surface of the annular streamof the second defined material flowing thereover, the thirdthermoplastic material applied downstream from the application of thesecond thermoplastic material; (f) applying an annular stream of afourth defined material to the inner surface of the annular stream ofthe third thermoplastic material flowing thereover, the fourth definedmaterial applied downstream from the application of the thirdthermoplastic material; and (g) applying an annular stream of the secondportion of the first thermoplastic material to the inner surface of theannular stream of the fourth defined material flowing thereover, thesecond portion of the first thermoplastic material applied downstream ofthe application of the third thermoplastic material.
 4. The process ofclaim 3 wherein the second and fourth defined materials comprise anadhesive.
 5. The process of claim 3 wherein the second and fourthdefined materials are thermoplastic polymers.